Attack is when some one goes to an base and loot them for resources and deals damage.
For looting a TH you get 8% of the total rescorces.
For Slios you get 5% of total rescorces.
For collecters you get what is in there.
A basic attack includes a mix of tower monsters and looting monsters. With harder bases, several waves are needed and sometimes a catapult is required to knock down tough towers.
There are many strategies involved with attacking an opponent. A good strategy is to view your opponents base and see if their silos are nice and fat, or if they have a lot of resource buildings. Then you should look for a point of weakness. A small gap in a wall, unguarded silos, or towers all by themselves are good targets. Try to stay from openings in the walls that lead down a long pathway. These are almost always filled with booby traps. Strategize and test ideas out. If at first you don't suceed, try, try again!